Changes in microbial biomass C, ATP content, soil phospho-monoesterase and phospho-diesterase activity following air-drying of soils

Abstract

The microbial biomass C in 20 grassland soils from New Zealand was estimated using the CHCl 3 fumigation method and from the rate of respiration using a modified substrate-induced respiration technique (SIR). Estimates were made before and after air-drying. The ATP contents and the activities of the enzymes phospho-monoesterase and phospho-diesterase of moist and air-dried soils were also measured. The fumigation method gave erratic results on dried soils and poor agreement with biomass C estimated by the SIR method. The SIR biomass C, ATP content, phospho-monoesterase and phosphodiesterase activities all declined after air-drying, the average decrease being by 39, 68, 38 and 29%, respectively. In general, biomass C determined by either method, ATP content and enzyme activities were positively correlated to the C content of the soil, but the relationships between the various indices were variable, and changed considerably after air-drying. The relationships were not sufficiently consistent for useful conversion factors to be derived. The modified SIR technique appears to have potential to estimate the biomass C of drier soils, particularly if the biomass at the time of sampling is required and rewetting of the soil is not desirable.